Dot multiplex



April 28, 1931; J. N. wHlTAKER DOT MULTIPLEX Filed Sept. 29,

1928 2 Sheets-Sheet l Qb QQQ n Q u NSA'.

April 28, 1931- J. N. WHITAKER 1,802,745

DOT MULTIPLEX Filed Sept. 29, 1928 2 Sheets-Sheet 2 I .E 'L5 ,53 53 56 a F/rf Fa 5 I 54 Y 2 do 57 xxi/viz F/r'l? .if /51 i] y 58 E. L une ,c

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25,5% H @VL H jj, j INVENTOR N. WHWAKER @www ATTORNEY Patented Apr. 28, 1931 PATENT FFlCE JAMES N. W-II'IAKER, OF YONKEB-S, NEVI YORK, ASSIGNOR TO RADIO CORPORATION OF i AMEBCA, fi CORPORTEON OF DELAVVABJE DOT MULTIPLEX- Application filed September 29, 1928. Serial No. 309,137.

The present invention relates to an imiroved method and means for multiplex communication in which a single radio or telegraph communication channel is used to send signals between t-wo or more points simultaneously while using a single transmitting apparatus. The invent-ion adapt-s itself particularly to the use of a single communication system foi' transmitting and receiving impulses which may be controlled by appropriate light modulating means for conveying impressions corresponding to varying intensities of lights and -shades for picture or television work, or for transmitting and receiving code signals in keying or automatic transmitter code work, or with slight modifications as regards the particular tone frequencies to be hereinafter described may be used for voice modulation, as, for example, in broadcast work.

The basic principles of operation whereby this invention is made possible has been disclosed by an application of Richard Howland Ranger tiled concurrently herewith as Serial Number 309,273, but this invention offers certain improvements and refinements over the said basicdot multiplex application, parti-cularly as relates to the current limiting means and the lmeans for keying the transmitterv when used in combination with the various tone frequencies suitable for multiplex work. iis in the above named Ranger application, the present invention oei's the advantage of turning a. simple telegraph or radio transmitter on oi off completely for each ofthe necessary signals or modulations which are utilized in forming the separate tones for multiplex work. Therefore, an object of my invention is to provide a multiplex communication system in which the various tone frequencies are limited as to their amplitude so that, when various tone frequencies are superimposed one upon another at various times during .the transmission, the amplitudes of the superimposed tone signal will not exceed that of the signal tone modulation signal.

A further object of my invention is to provide, in a manner to be hereinafter set forth, a. means whereby a transmitting apparatus associated with my multiplex system may be cuits and which will finally resolve itself intoy a iiiial squared wave or series of pulses of a. diiferent frequency for each channel. K rlhe greatly distorted wave form, it will be noted, produces a wave of great amplitude but of extremely short time duration so that the lwaves from the various tone channels will not overlap each other to any great extent, thereby insuring ease in distinguishing one tone from another in the receiving system.

@ther objects of my invention are to pro--V vide, in a manner hereinafter set forth, a multiplex communication system which is efticient in its operation, convenient to install,

inexpensive, substantially fool proof and one which may be provided by merely making a very few chang-es to the existing transmitting and receiving systems of communication now provided for use in transmitting and receiving only one message at a time.

rllhe invention according to its preferred form has been illustrated by the vaccompanying drawings which, when taken together with the description and claims, forms a clear and concise description of one embodiment which the invention may assume, although it is to b-e distinctly understood that various modifications may be made which fall fairly within the spirit and scope of the invention as set forth in the claims. These modifications will at once suggest themselves to anyone skilled in the art to which my invention relates.

By the accompanying drawing, Fig. l represents one form of transmitting apparatus;

Fig. 2 illustratesin further detail a portion of one channel of the transmitting apparatus of Fig. l, and shows particularly the three vacuum tubes used in the production of the squared wave form from the distorted peak wave produced by the tuning fork;

Fig. 3 represents a receiving system which may be used in connection with the transmitter of Fig. 1;

Fig. l diagrammatically illustrates the relationship between the amplitude of the various tone impulses and the time separation thereof for each channel; and,

Figs. 5a to 5cl, inclusive, represent the various wave forms which are produced; in which Fig. 5a' illustrates the greatly distorted or peaked wave produced by the strenuous swing of the tuning fork; Y y

Fig. 525 illustrates the Wave appearing in the output circuit of the second vacuum. tube in each signal channel;

Fig. 50 illustrates the wave appearing in the input circuit ot the third vacuum tube in each channel and,

Fig. 503 the wave form appearing as a squared wave for keying the radio or wire transmitter and the wave form which appears in the output circuit of the third vacuum tube of each channel; and,

Fig. 6a illustrates the input to the transmitter tube 19; and

Fig. 6b illustrates the transmitter output.

Referring now to the drawings and considering first the various wave Jforms shown by Fig. 5 together with the circuit illustrated by Fig. 2 particularly, the portion of the circuit to the le'lt of the dotted line of Fig. 2 is the vacuum tube circuit and fork driving means. The tuning fork 1 is maintained in vibrational operation through the coil windings 2 and 3 and, due to the strenuous swing of the fork tines back and forth, periodically influences the potential applied to the grid member oi the vacuum tube A. The wave thus produced by the swing of the orx tines is i-ed through the condenser 5 to the grid member ot the tube (3. The grid of the tube 6 receives a high positive bias from the battery 7 so that only the negative peaks or" the wave form illustrated by Fig 5o becomes et'- fective and the wave torni appearing in the output circuit of the tube G then assumes t form illustrated by Fig. 5o. rlhis wave form is then fed through the primary of the transformer and the resultant which appears in the secondary assumes the ffenerll 'form of the wave which is diagr-nmnatically illustrated by Fig. 5c.

The tube 9 to which thev wave form 5c appearing in the secondary of the transformer 8 is applied, is biased by means of the battery 1() to a point, for example, 20 volts beyond cut oli so that the negative portion of the wave is rendered ineii'ective and only a small section of the positive wave takes effeet. Then the positive portion of the wave 'form shown by Fig. 5c reaches the grid member of the tube 9 it causes maximum plate current to be drawn in the plate circuit of this tube long before the peak voltage is Since all channels of the system are iden-v tical, only the channel above described and shown by Fig. 1 as channel A has been discussed in any detail but from an examination oit' Fig. 1 it will be noted that the only dierence in variation between the channels A, B, and C, respectively, is in the frequency of the forks used for producing the distorted wave forms.

The condenser 5 is of very small capacity and, as has been previously stated, the bias battery 7 is of such a high positive value that only the negative peaks of the waves are eiiective. The biasing battery 10 is of such a. value that the tube 9 is negatively biased to a point below cut off as mentioned previously.

New referring to Fig. 1 and continuing the description thereof from the point where it differs from Fig. 2, 11 illustrates a key or other preferred form of apparatus which may be used to modulate the radio or line transmitter but for simplicity of explanation it will be assumed that 11 represents a telegraph key for keying the channels A, B, and C, respectively. Consider, Iior example, iii-st that the key 11 associated with the channel A is closed, under which condition channel A becomes effective and will act upon the limiting` device generally designated as 12 and which includes a vacuum tube 13 with a resistor 14 connected in series with the plate current supply. The resistance 14 is of such a value that a single pulse of energy in the circuit of the tube will swing the grid members of the amplifying tubes 15 and 16 to a minimum value from a maximum and therefore should the occasion arise, as it periodically does, that several energy pulses or tone pulses in the various signal channels occur simultaneously, the plurality of pulses can have no greater effect upon the grid members of tubes 15 and 16 than did the single energy pulse above described in connection with channel A. Therefore, a limiting action occurs. it will be noted from the disclosure that the tube 13 draws a maximum amount of plate current during the signal oit periods and, therefore, :ith each signal pulse from a single channel or from a plurality of channels the gridswings negative to cut oit, due to the voltage drop occurring across the resistance 17. Asabove explained, it is seen that each pulse swings the plate current of the tube 13 from maximum to complete zero, and consequently if two pulses occur at the same instant, causing twice the same drop across the resistance 17 there can be no change in the amount of plate current drawn in the plate circuit of tube 13 over the amount drawn when only `a. sing'le pulse occurs due to the fact that a single pulse causes the plate current to drop to Zero which of course is the least potential to which it can possibly drop.

Now referring to Fig. 4 t-he output as Well as the input energy of the tube 13 is diagrammatically illustrated. Assuming that all the keys 11v associated with the various signal channels A, B, and C are closed and act upon the limitor, the `input to the limiter appears as shown by the first energy pulse with the various signals from each channel superimposed one upon, the other and shown by dotted lines, While the output energy from the limitor appears as shown in solid lines. The squared Wave forms of constant amplitude are impressed upon an amplier which may be, if desired, of the character illustrated by 15 and 16 which is of the customary direct current amplifier form and needs no particular eX- planation. This amplifier has its output circuit energy fed to a transmitting apparatus of the character generally designated as 18. The transmitter may consist of one or more amplifying tubes 19 associated With a crystal controlled roscillator or frequency amplifier Which is connected across the grid circuit of the amplifier. The input circuit of the amplifier includes a radio frequency choke coil 2O and a pair of grid resistors 21 and 22 having one end of each connected together. A blocking potential is connected across one of the said grid resistors nearest the amplifying triode so that a negative biasing potential may be applied to the grid and so that the positive potential point is connected furthest away from the said grid member. This positive potential bias may be, if desired, of the same potential as is used to influence the plat-e members of the vacuum tubes 9 and 1S and for the purpose of illustration, let us assume that the grid of amplifier 19 is biased 300 volts negatively. The amplifying tube system 15 and 16 is associated with the opposing resistor 2 2 in' a manner such that its positive potential is applied to the end of the resistor 22 next to that of resistor 21 and its negative terminal is connected With t-he filament member of thesaid amplifier so that, for purpose of illustration, supposing there is current flowing in the output circuit of the amplifying tubes 15 and 16 assume that a voltage drop of 220 volts occurs across the resistor 22.

This voltage drop Will oppose the SOO volt drop assumed in the resistor 21 and the grid member of the amplifier 19 will. be negatively biased by volts which, for a transmitter tube of the character known as the UX 250, Will permit operation thereof but, when the tube is biased to a point 300 volts negative it will be blocked and no energy vfed over accompanying drawing and may consist of an amplifying-triode 24, a piezoelectric crystal 25 connected across Vthe input circuitof the amplifying triode Vand shunted by a choke coil and biasing battery for the grid member of the triode amplifier. Pla-tepotential Vfor amplifier 24 is suppliedy from a plate battery or from the source 26 and is fed through a radio frequency choke coil 27 to the plate member of the amplifier y24. The output of the tube 24 is passed through capacity l28 to the circuit 29 and then iinpressed upon the input circuit of `amplifier 19 through a capacity coupling.

Plate energy for the amplifier 19 is supplied from a battery 30 and passes through a radio Yfrequency choke coil 31 to the plate. The output energy from` the tube 19 is fed through a capacity element 32 by which Ait is transferred to .the oscillatory ycircuit 33. Connected to the oscillatorycircuit 33 is an antenna member 35 grounded or provided With a counterpoise 36,' as desired, from which energy maybe transmitted.

It is thus seen that, with the forks `1 each being driven so as to provide a different tone frequency and from a consideration of Fig. 4 vshowing the tone output, When the `grid member of the tube 19 is periodically and at a definite rate swung positive from a vblocked negative value, Vthe transmitting system is keyed and unkeyed at a vrate corresponding to all of the separate tones and that energy is thus. fed out throughthe antenna system atV this particular rate; When there is an interruption of the tone frequency being supplied, due to spacing between the impulses, no energy .is fed 'over the radio transmission channel or over the Wire line. 1t must be stated here that the length of each of the individual tone pulses is, of course, .a function of the number of message channels,

and as the number'is increased'ithe'impulses rWill be vshorter and similarly'longer for a decrease in the number .of channels.

Signals transmitted from `the antenna system 35, or, if desired, an appropriate line transmitting system, may then bereceived by an appropriate receiver, such asthe antenna system 50 grounded "at 51. The receiving system suggested by this application is a substantial duplicate ofl that described in the above referred to'patent application of Richard Howland Ranger, filed concurrently herewith. The receiver per se forms no essential part of this invention only in 4so Vfar as it cooperates With the specific typeoftransmit- ,ter illustrated and described in connection With Figs. 1 and .2 ofthe drawing. HOW- ever, Vfor the purpose )of explaining this invention it. may be stated briefly that the receiving system Where the invention isapplled to radio transmission and reception includes the antenna and a receiver 52 of any desired and appropriate character Which may include radio frequency amplifiers, a detector, and audio amplifiers, or, if desired, may be of the heterodyne type.v Output energy from the receiving sys-tcm conventionally shown as 52 is then direct-ed'to a series of filters 53, 54 and 55, designated further as Fa, Fbiand Fc. These; filters pass the particular tone frequency of the channels"A, B and C of the transmitting system and, as herein shown and described, may, for instance, pass respective frequencies of 85() cycles, 1190 cycles, and

- 1530 cycles Which are all harmonics of a basic frequency herein assumed to be 34 cycles. It will be noted that in the case described each of the tone frequencies which are passed by the filters are of an audible nature and these particular frequencies are then impressed upon the receiving elements which, as shown, may be the phones 56, 57 and 58 although, it is to be understood, that siphon recorders, marking pens, electric controlled gas recording valves, or other equivalents may be substituted for the phones for the purpose of obtaining visible records, or, if desired, light recording means such as glow lamps or the like may readily be substituted and deemed the full equivalent of the phones. If desired, capacity elements 59, 60 and 61 may be shunted around the phones as shown although, it

' is to be understood that this is not essential.

I have shown by Fig. 6a the form of transmitter input which,it will be noted, appears as an energy pulse of relatively long time duration which, when operating the transmitter, Will appear in the output and be trans- `mitted in the antenna after a fashionA shown by Fig. 6b. From this consideration it Will be seen that While the received signals appear in` the phones or other appropriate recorder as a tone they are in reality a series of high speed dots or pulses and composed of groups of radio frequency Waves.

Bytliis disclosure Ifhave ldescribed only one of manyjmodifications or forms Which my invention is capable of assuming but, it is to be understood, that I may substitute various equivalent forms such as Will readily suggest themselves' to those skilled in the art and I, therefore, desire to be limited in the modifications of the invention which I may choose only in so far as the scopeof the hereinafter appended claims necessitates.

Having now described my invention. what I claim and desire'to secure by Letters Patent is:

1. In a single channel multiplex communication. system, means `for producing a plurality of individual tone frequencies, a transmitter, 'means for successively and continuously supplyingall of said individual tone keach cycle of all frequencies to said transmitter,and means for limiting the strength of the current impulses corresponding to each of said individual tone frequencies applied to said transmitter whereby Wheny a single cycle of said individual tone frequency is superimposed upon a single cycle of the other tone frequencies the output. of the said transmitter is no greater than for a single cycle of any of the individual tone frequencies.

2. A multiplex communication system including means for producing a plurality of individual tone frequencies, a transmitting system for transmitting impulses varied in accordance with each of said tone frequencies, and means for successively keying said transmitting system for each cycle of all of said tone frequencies. Y

3. A system for transmitting a plurality of messages over a single communication channel Which includes means for producing a plurality of tone frequencies each of which is a predetermined harmonic of a predetermined basic frequency, means for synchronizing each of said individually produced tone frequencies for exactly superimposing one cycle of each of said individual tone frequencies upon a corresponding cycle of each of the other harmonic tone frequencies, means for limiting the current strength of said superimposed cycles of all of said tone frequencies, a transmitting system, and means provided by said limiting means for turning said transmitter on and 0E completely for eachcycle of all of said individual tone fre- ,f

quencies. Y c

4. rlhe methodV of simultaneously transmitting a plurality of messages over a single communication channel Which includes `enerating a plurality of individual tone frequencies, synchronizing'each of said individual tone frequencies so that Where successive cycles of the individual tone frequency overlap they are superimposed one upon the other exactly, limiting the current strength of the superimposed cycles of said individual tone frequencies, and keying a transmitter for of said individual tone frequencies. Y

5. The method of multiplexingsignals over a single communication channel transmitting and receiving system Which'includes generating a plurality of audible tone frequencies each corresponding to a chosen harmonic of a predetermined basic frequency, synchronizing each of said harmonic tone frequencies for exactly superimposing predetermined cycles of the said tone frequencies in accordance with a rate corresponding to the chosen harmonic of the predetermined basic frequen- 'i1-f cy, limiting the current strength of the superimposed cycles of the combined tone frequencies to a value equal to that of an individual cycle of a single tone frequency, keying a transmitting system once for each cycle of all of said tone frequencies, 'receiving the transmitted frequencies as a series of radio frequency pulses of a duration corresponding to the time period of one cycle of each of said individual tone frequencies, and filtering the received signals to produce audible tone frequencies corresponding to the generated tone frequencies.

6. A multiplex communication system including, means for producing a plurality of individual tone frequencies each bearing a predetermined relationship to a predetermined basic frequency, a current limiter for limiting the intensity of each of the individually produced tone frequencies, a transmitter for transmitting energy controlled in accordance with each cycle of each of the produced tone frequencies, and means for completely keying' said transmitter system for each cycle of all of said produced tone frequencies.

.7. rlhe method of multiplexing signals over a single communication channelwhich includes producing a plurality of individual tone frequencies, amplifying the energy produced, connecting individual tone frequencies of a number corresponding to the number of signals to be simultaneously transmitted with the amplifying system, limiting the strength of the tone frequencies to a predetermined Value, transmitting impulses controlled in accordance With said tone frequencies of limited value, and completely keying the transmission for each cycle of all of the produced tone frequencies.

8. The method of multiplexing signals over a single communication channel which includes generating a plurality of audible tone frequencies each bearing a predetermined relation witli respect to a predetermined basic frequency, synchronizing each of the said generating tone frequencies for exactly superimposing predetermined cycles thereof, limiting the current strength of the superimposed cycles of the combined tone frequencies to a Value equal to that of each individual cycle of each of the produced tone frequencies, and keying a transmitting system once for each cycle of all of said tone frequencies.

JAMES N. WHITAKER. 

